Iran has for decades practiced what critics call hostage diplomacy, a policy of detaining foreigners and dual nationals to leverage them for prisoner swaps and the release of frozen funds. In the aftermath of the 12-day war with Israel and the United States, Iran is once again targeting Americans.

At least four Iranian Americans — two men and two women — are in Iranian custody, according to human rights groups, lawyers and Hostage Aid Worldwide, a nonprofit organization that was founded by former hostages to aid families and that is in touch with the current detainees’ friends and families.

Three of the Americans are in jail, and one has been barred from leaving the country, they said.

The detentions are likely to increase the already tense political climate between Tehran and Washington after the United States joined Israel’s attack on Iran and bombarded and severely damaged three of its nuclear sites in June.

Nuclear negotiations with Washington have not resumed since the war in June, but Iran’s foreign minister, Abbas Araghchi, said this past week in an interview with local news media that he and the U.S. special envoy, Steve Witkoff, have been communicating directly through text messages.

President Trump has said that he would not tolerate countries’ wrongful detention of Americans and that their release is a top priority for his administration. Mr. Witkoff’s office did not respond to a question on whether the detention of dual American citizens was brought up in communications with Mr. Araghchi.